Rainstick - a musical percussive instrument made from a hollowed-out stem of a woody cactus, chiefly Echinopsis chiloensis and Eulychnia acida from Chile, filled with small pebbles or sand and sealed at both ends. Spines are inserted to make baffles within the hollow stem. Large numbers of rainsticks are exported from Chile to the United States and Europe as curiousities.

Raphide - (Raphis) a needle-shaped crystal found in some plantcells, usually a crystal of calcium oxalate as a metabolic byproduct. The irritatingly sharp crystals discourage predation by herbivores.

Rayflower - one of the outer irregular florets, typically with a single tongue-shaped corolla, on flower heads of some species of Compositae (Sunflower family).

Receptacle - enlarged apex of a flower stalk bearing the parts of a flower or the florets of a composite flower. In ferns, a main stem on which sporangia arise.

Regular flower- a radially symmetrical flower in which similar parts are equal in size, form or extent of fusion.

Reniform - an oval with the ends curved around in the same direction like a kidney.

Residual - a property of a substance, such as a pesticide, that allows it to remain in an area for a long time.

Respiration - a process of releasing energy by oxidising molecules such as glucose to carbon dioxide. Respiration produces useful reactive molecules, such as ATP (adenosine triphosphate) which activates many cellular processes by transfer of its terminal phosphate group.

Rib - a leaf vein. A ridge of plant-tissue thicker than its surroundings. A raised surface running vertically, sometimes spiraling, and in the Cactaceae bearing a row of areoles along its summit. Morphologically, a rib is composed of a continuous line of fused tubercles, which in some cases may be discerned as bulges in the structure.

Ricin - an extremely toxic protein found in the seed of the castor oil plantRicinus communis, a member of the Euphorbiaceae, grown commercially for the oil content of the seed and sometimes grown as an ornamental garden annual. Less than a milligram of ricin injected into the blood stream can be fatal.

Riparian - of plants growing along the side or on the bottom of a watercourse, or in other aquatic areas.

RNA - ribonucleic acid- an extremely long single-stranded linear polymer, made (transcribed) as a secondary copy of the gene sequences in DNA and also found as the primary genetic material in some viruses e.g. tobacco mosaic virus.
RNA is found in cells as three basic types:

rRNA - ribosomal RNA associated with proteins in ribosomes, the sub-cellular mechanisms responsible for building proteins from the mRNA sequence

Rheotropism - bending, growth or movement of a part under the influence of a water current.
See also: Tropism.

Root - a structure usually providing a plant with anchorage and a means of absorbing water and nutrients from the substrate. Many Cactaceae have a widespread shallow fibrous root system, in addition to any taproot or tuber. This is an adaptation for scavenging water before it evaporates. See also aerial root.

Root hair- a trichome on the root epidermis made from an extension of an epidermal cell which assists with absorption of nutrients.

Rosette - a tight cluster of leaves that are arranged around a central apex and grow outward to form a radially symmetrical pattern.

Rubber - ( caoutchouc) an organic (polyterpene) component of the latex produced within the bark of large trees in the family Euphorbiaceae (spurges) notably the Pará rubber tree (Hevea brasiliensis) native to the Amazonian rain forest in South America. Seeds from the rubber tree were sent in 1876 to Kew gardens, London, England by Henry Wickham, a local planter. Following germination of the seeds in the tropical herbarium at Kew, seedlings were exported to Ceylon (Sri Lanka) and a year later sent from Ceylon to Singapore where the method of tapping the tree for latex was developed. This avoided felling the tree to obtain latex. The rubber tree is grown commercially in Malaysia, Thailand, Indonesia, and Sri Lanka but only to a very limited extent in South America.
The latex from the rubber tree is an aqueous emulsion, also containing various proteins, and is generally coagulated by heat to produce natural rubber. Most of this is vulcanised by heating with sulphur to cross-link the polymer chains and harden the product.